The present invention relates to improvements in digital vertical synchronization systems for television receivers, sometimes called "vertical countdown systems." It relates particularly to apparatus in digital synchronization systems for determining whether the system should operate synchronously or asynchronously.
Digital vertical synchronization systems such as those described in Merrell and Hendrickson, U.S. Pat. No. 3,691,297 and Merrell, U.S. Pat. No. 3,916,102, both assigned to the Zenith Radio Corporation, the assignee of this invention, provide locally generated vertical synchronization pulses having relatively constant shape, phase and amplitude. Such locally generated sync pulses, instead of integrated vertical sync pulses, are applied to the vertical sweep generator for controlling raster scan.
The basic system described in those two patents included a source of clock signals applied to a line counter. The line counter counted 525 clock intervals corresponding in rate and in number to 525 lines of the standard NTSC raster scan. The received vertical sync signal was integrated and compared with a 525 count signal to determine whether the line counter was properly in phase with the received vertical sync system and in response thereto, an up/down counter changed state toward one of two extremes. One such extreme represented a maximum confidence condition wherein a succession of properly phased internal clock signals had been produced. In such maximum confidence condition, the line counter continued without alternation of its time relationship to the received vertical sync signals. However, when a certain succession of internally developed vertical sync signals did not coincide with the integrated vertical sync signal, so that the counter reached its other extreme condition representing a minimum confidence level, then the system reset the counter to be in a proper phase relationship with the integrated vertical sync signal.
In that system, a problem arose when the television receiver accepted input signals from sources other than over-the-air television transmissions, such as cable television. Over-the-air broadcasts conform, in the United States, to NTSC standards and provide vertical sync pulses at a specified rate. Those transmitted vertical sync pulses are referred to as "standard," that is, occurring at the designated NTSC rate. Cable television systems and other sources sometimes communicate vertical sync pulses which are not at the NTSC specified rate. Such signals are referred to as nonstandard. To conpensate for such nonstandard signals, the digital vertical sync system described in the above-mentioned Merrell U.S. Pat. No. 3,916,102 included means within the phase lock system for maintaining the counter at its minimum confidence level during reception of nonstandard signals. As a result, the 525 pulse counter was reset with each received integrated vertical sync signal.
A "mode recognition" circuit was included in the prior art system to determine whether the digital vertical sync system should enter the standard mode or the nonstandard mode. Inasmuch as NTSC broadcast signals include certain equalizing pulses during specific times within the vertical interval, the system counted the number of equalizing pulses received during a specfic interval. If nine or more equalizing pulses were counted by the mode recognition circuit, the synchronization system operated in a standard mode. On the other hand, if fewer than nine equalizing pulses were detected by the mode recognition circuit, the synchronization system entered a nonstandard mode. This was done on the premise that if an NTSC signal was being received, it was standard. Conversely, if a non-NTSC signal was being received, nonstandard signals were most likely.
Such a system has not been completely satisfactory due to the increasing use of devices such as consumer video tape recorders with playback speed selection. When a consumer records a broadcasted television program, equalizing pulses during the vertical interval are recorded. When the tape is replayed, the mode recognition circuit of the digital vertical synchronization system is tricked: it counts nine or more equalizing pulses and enters the standard mode. The vertical synchronization would be proper, in that case, if the video taper recorder plays back at the proper speed and without jitter, wow or flutter, but not using fast, slow or still motion. However, when the playback speed varies, the mode recognition circuit will not detect a change and the television receiver will display an image which rolls or loses sync and is generally difficult to view. One solution to that problem is to force the synchronization system into the nonstandard mode by a manual adjustment ("forced mode"). This manual adjustment, however, is not readily practicable for home consumers.
Therefore, one object of the present invention is to provide a digital vertical synchronization for a television receiver system which properly operates with either standard or nonstandard vertical sync signals.
Another object is to provide such a synchronization system which automatically enters a nonstandard mode if a television signal is inputted to the receiver having vertical sync signals at a non-standard rate, regardless of the number of equalization pulses in the vertical interval of such inputted composite signal.
When a digital vertical synchronization system operates in a nonstandard mode, its immunity to noise is substantially degraded as compared with its immunity during the standard mode. Accordingly, a further object of the present invention is to provide a digital vertical synchronization system as set forth above which improves the noise immunity of the system.